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Home , Glycolaldehyde, Homochirality, Panspermia, RNA world

Lattimer, AST 248, Lecture 13 – p.1/20 rganic only certain ation requires a formation naturally for polymerization. the first self-replicators s, or , such as oteins and nucleic acids. e reproduction. It is widely, ed protobionts, that had gs. This relies heavily on . 2 H 2 N+CN 2 O 5 H 2 C . 5 , O 5 −→ energy 10 N 5 H O 5 H 2 5 C C → → CO 2 Production of (an ) 3HCN+2H Production of (a base): 5 HCN Production of (a ): 5H • • • amino acids and . occurs if phospholipids are present. Bernal showed that -like materials could serve as sites chemical characteristics different from theirthe surroundin formation of a semi-permeablematerials membrane, to one flow that one allows wayliquid or with the a other large through surface it. tension, Droplet such form as . Membrane but not universally, believed that RNA-like were — the RNA world hypothesis.self-replicators. They may have been preceded by ino 1. the synthesis and accumulation of small organic 3. the concentration of these molecules into droplets, call 2. the joining of these monomers into polymers, including pr 4. The origin of heredity, or a means of relatively error-fre Chemical Theory of 's Origins the Lattimer, AST 248, Lecture 13 – p.2/20 organics. lk of the solar is delivered s volatile material, lead to fractional em. atile materials are much as teorite. rate was much larger. of the average. 3 O. At present-day rates, a billion years is needed to deliver − 2 10 C in ’s , but 4 billion years ago, the delivery amounts of C of concentrated in the outer solar system. Although there isnebula, as the high in the inner solar system have especially C and H material, as indicated by their presence in the Murchison me In the standard model of the formatio of the solar system, vol Ices are similarly much more abundant inMeteorite the and outer solar impacts syst could deliver much of the Earth’ Some simple organic materials would have been included in th A reducing atmosphere on the would generate more nearly all other heavy elements combined in the Sun and the bu Acquisition of Organic Material and Water • • • • • Lattimer, AST 248, Lecture 13 – p.3/20 tein”. ence of RNA eriment there hemical ing, suggests an d theory, De Duve which natural p until now, but new step, from very simple ontaneous generation → → atoms → replication. Example: Wächtershäuser’s iron-sulfur worl thioester theory. But can’t explainreactions. the Thermosynthesis high world, specificity involving of thermal c ATP-like cycl that promotes bonds: the “First Pro “-first” mechanism “-first” mechanism Primitive metabolism provides environment for later emerg • • selection could act Synthesis of nucleotides Polymerization of nucleotides Incorporation of a self-copying gene into single cells upon Origin of Genesis of the mechanism

Abiogenesis • • • • • evidence has become evident: Self-organization leads to more complexBig structure Bang Crucial questions which did not have experimental answers u Pieces are now coming together to supportMinimal plausibility number of of sp seems to be about 206 in theory, inEvidence exp suggests that this complexity has evolved, step by seem to be 387 essential genes beginnings • • • • • as reducing Lattimer, AST 248, Lecture 13 – p.4/20 egligible after all. sphere and pointed s highly e Earth’s building . More recent evidence is that H 2 Miller & Urey experiment). cf. ( oxidizing

Monomer Production: to a neutral atmosphere dominated by CO Later research cast doubt on the existence of a reducing atmo opposed to escaped very slowly on early Earth and its abundance wasn’t n Supported by evidence from chondriticblocks. Discovery which of wer highly reducing conditions near hydrothermal vents and in volcanos may make this debate irrelevant. Step 1 is possible in the early Earth’s atmosphere if it was wa Energy sources to drive initial chemical reactions available from UV solar radiation, radioactivity, electrical discharges (lightning), cosmic rays and solar wind (Earth’s magnetic field not yet formed). Volcanic and vent energy available near hydrothermal vents. • • 2 . N 2 Lattimer, AST 248, Lecture 13 – p.5/20 , , a 3 onreactive N d to different crystal expanding gases; cite bonds strongly destruction. Example: ng. s are abundant. interaction. Example: d have occurred near passed over metal surfaces can bind and generate NH 2

The Role of Minerals common minerals developing microscopic pits from weatheri Protection and concentration Minerals acted as hosts, protecting them from dispersal and Support Surfaces act as support structures aiding accumulation and Selection Many minerals have crystal faces that are mirror images. Cal is important, but most of it is in the atmosphere as n volcanic rock containing many small air pockets formed from clays. with amino acids, and left- and right-handed amino acids bon faces. and H valuable source of nitrogen forhydrothermal biological vents reactions. where Coul iron oxide and iron sulfide surface • • • • Four key roles minerals could have played: Lattimer, AST 248, Lecture 13 – p.6/20 00 bases in eparted by 1 mm or ea of a football field. ymerized into Andreas Trepte, translated by User:Itub; Wikipedia 4 and Al O 4 Clays and Polymerization more, enabling dehydration. length have been produced inpre-cells, laboratories; sometimes lipids containing can short be RNA pol strands. Clay structure is that of alternating negatively charged sheets of Si O Clays are extremely common on the Earth and . Charged layers and cations provide multitudinous sites for monomers toWater stick. can easily flow through the structure as the layers are s A cubic centimeter (thumb-tip) of clay hasMany the peptide net bonds surface are ar catalyzed by clays; RNA strands up to 1 tetrahedra separated by positive cations (Ca, Na, Fe, or Mg). • • • • • • Lattimer, AST 248, Lecture 13 – p.7/20

Clays Lattimer, AST 248, Lecture 13 – p.8/20 ce with hydrophilic ilic head and a a membrane. n water.

Droplets and Boundary Layers heads in the water, creating a single (or mono-) layer, i.e., Certain materials are ambiphilic: they have a polar hydroph Ambiphilic molecules added to water tend to stay on the surfa hydrophobic tail. Hydrophilic materials can be dissolved i • • How does self-assembly occur? Formation of spheres, or micells, permits surface area and free energy reduction. In sufficient concentrations, ambiphilic molecules will form a double-layer structure, or bilayer. Spheres, or bilayer vesicles, will form. Lattimer, AST 248, Lecture 13 – p.9/20 3 2 idual molecules 2 · 3 2 π · 2 3) π/ 16 π 32 π/ 4 (4 1 large drop www.daviddarling.info/images 3 1 2 · 3 1 π · 1 3) π/ 32 π 32 π/ 4 (4 8 small drops ) 2 ) ) 2 3 ) 3 Properties radius (mm) total volume (mm volume per drop (mm Droplet Formation total surface area (mm surface area per drop (mm High surface tension of water leads to large drops, not indiv Polymeric clumps () observed Phospholipids self-assemble into films forming semi-permeable membranes Concentration of polymers Existence of for growth Fission forms daughter drops Limited raw material, growth enzyme Random inheritance of important enzymes Keys: Autocatalytic polymers, systematic inheritance • • • • • • • • • Lattimer, AST 248, Lecture 13 – p.10/20 organic molecules and membrane-bound bubbles primitive cells

Droplet Formation Lattimer, AST 248, Lecture 13 – p.11/20 os/106/unit04/3a.protobionts.html The droplet, consisting ofcontains protein the and enzyme , phosphorylase.diffuses -1-phosphate into the dropletthe and enzyme. is polymerized The to starchit, migrates and to by increases the volume wall, of thickens droplet.

Droplet Growth instruct1.cit.cornell.edu/courses/biog105/pages/dem The enzyme, phosphorylase, polymerizes glucose-1-phosphate to starch. A second enzyme, amylase, degrades the starch to . Dropletsenzymes containing do both not growdisappears because as the fast starch asMaltose it diffuses is back made. into surrounding medium. Lattimer, AST 248, Lecture 13 – p.12/20 tides; ble scenario exists es; eplicated and evolved more efficient. iruses. peted the RNA world. l wreak havoc in the modern

Why RNA Might Have Been First RNA nucleotides are more easily synthesized thanDNA’s DNA greater nucleo stability argues it took overRNA some probably of evolved RNA’s before rol most because no plausi The molecule ATP is closely related to a of RNA. where proteins can replicate without RNA or DNA. • • • • without specialized proteins. Eventually, RNA became capable of transcribing DNA which is This suggests a simpler RNA world existed once, in which RNA r RNA can create DNA, as is illustrated by the example ofNatural selection retrov led to the DNA + protein world which outcom Are a dark legacy of our ancestors that can stil world? Lattimer, AST 248, Lecture 13 – p.13/20 e) that interacts ng DNA into RNA. . variety of shapes le in protein s. information ns, necessary for life’s nt forms implying they

RNA World Hypothesis functions, is useless with surrounding molecules. independently formed DNA and methods of transcribi replicate without proteins catalyze all steps of protein synthesis Proteins were not first, they can’t beDNA catalyzed gene without information gene without catalysis, provided by protei New pathways recently found for self-assembly RNA, not protein, enzymes called play a central ro RNA is also an important catalyst forPrimitive the RNA synthesis sequences of can new evolve under RNA abioticUnlike condition double-stranded DNA, single-stranded RNA can take a RNA thus have both a (sequence) and (shap Protein in and have differe synthesis, although proteins also speed up the process. specified by their nucleotide sequences. • • • • • • • • • • • Prebiotic life has an RNA forerunner if it could: RNA today does not have these properties. However Evidence for the “RNA World Hypothesis”: Lattimer, AST 248, Lecture 13 – p.14/20 pyramidine Powner, Gerland & Sutherland 2009 ribose

Abiotic Nucleotide Synthesis cyanoacetaldehyde Lattimer, AST 248, Lecture 13 – p.15/20 that were olecules like lates holding a enetic molecules could ing of specific es this today). e and replicate rudimentary selection. tion, but because thier curately reproduced. n leads to more stable minate the population of

What would happen: S to provide energy. 2 have been short, -like sequences. enzymes for RNA replication. H properties were heritable, they could be acted on by natural information; being double-stranded it is more stable and ac Reactive nucleotides made random polymers, and the first RNA Inside protobionts, some amino acid polymers could have had Under certain conditions, some RNA sequences are more stabl RNA-directed protein synthesis may have begun as a weak bind Some RNA molecules may have produced short amino acid chains Other RNA sequences may have enabled the use of high-energy m A protobiont with self-replicating, catalytic RNA would do The first protobiont would have only limited genetic informa (occasional copying errors) and natural selectio Natural refinement to replace RNA by DNA as the repository of g catalytic properties, aiding in RNA replication. faster and with fewer errors than other sequences. amino acids to bases alongfew RNA amino molecules acids which together served long as enough temp for them to link (rRNA do molecules. and faster replicating varieties. • • • • • • • • • • Lattimer, AST 248, Lecture 13 – p.16/20 be simple and chinery for replication

Genetic Takeover Hypothesis provided by environment RNA, nucleotides too complex, evolved from simplerInterdependency systems evolved by means of scaffolding First contained information only; material and ma Organic molecules too varied, reversible bonds too weak, to Simple inorganic self-assemblers with strong reversible bonds exist: soap bubbles, clay crystals Growth naturally controlled by supersaturation Life is information: crystal defects self-assembling • • • • • • • • Information: control of environment (supersaturation, raw materials?) • Evolution () by direct genetic action • Advantage gained by going to indirect genetic action • Organic polymers have advantage of efficiency, flexibility, size • Possible links: • can control supersaturation • Amino acids control concentration of metal (Al, Mg) • control consistency of solutions • Gradually, structure & genetic information transferred to organic polymers • Organic polymers take over. Efficiency and self-assembly work against inorganics

Lattimer, AST 248, Lecture 13 – p.17/20 Lattimer, AST 248, Lecture 13 – p.18/20 r billions of years. Mars; even more osion. if life is discovered dreds of millions of s or not. eorites and re, no other location in a decade. ure to solar wind and terms of travel times. th. yer on a ; lanetary space. llions of years before onditions arose first, and ike or Mars, then it e by migration. But this , both in terms of number

Panspermia or Exogenesis sensitive, volatile material within is protected. implies organic material forms and survives in harsh interp years, points to possible long-term survivability. Over 3 dozen known meteoritesbelieved believed to to have have originated originated on on the Moon. cosmic rays in between melts or destroys a relatively thin la migrate from there. Mars had suitable conditions before Ear Once an outlandish idea, presence of organic material in met Discovery of , which might remain dormant for hun Rocks can be ejected into spaceEarth, as Moon, a Mars result and of Venus an have impact’s been expl exchanging material fo Original blast, fiery passage through atmosphere, and expos Vast majority of projectiles from Venus or Mars will orbit mi Migration from other stellar systems is extremely unlikely If life is extremely difficult to form, then perhaps it got her If life is easy to form, life would originate where suitableIf c it is possible for Earth life to survive on another planet l landing again on a planet, but 1/10,000 of them will landof with such meteorites (no known extra-solar meteorites) and in just moves the problem ofknown origin where to formation another might place. have Furthermo been easier. might well be there alreadyon because Mars of we migration. have This to means carefully consider whether it is indigenou • • • • • • • • • • † C”. ◦ roscopic Lattimer, AST 248, Lecture 13 – p.19/20 claimed to be s: not , N and O have t positive results s from high about 9 Gyr. strial DNA. nted. ossible. Recent llions of years. een LBH 3.9 Gyr ago ago. On the other ght by many to be too water up to 300 de our Solar System. in our Solar System (Mars, structures resembling terrestrial microfossils, but thou altitudes (up to 40 km) that are common terrestrial organism Red Rain of Kerala: Analysesextraterrestrial, (2003-6) which of “reproduce dust plentifully” yielded even inMeteorite “ ALH84001 from Mars was shown (1996) to contain mic small to sustain life. Furthermore, abiotic origin is very p Several studies (2000-2003) claim to obtain inconsistent with . Claims of bacteria inside meteorites (2001)Lingering with suspicions non-terre by some Viking lander researchers tha research into makes this find interesting again. in the life experiments were not false-positives.

Circumstantial Evidence for Panspermia • • • • • been present for 12.7 Gyr. This window for exogenesis is thus and the earliest evidence forhand, life the on age the of Earth, the 3.5-3.85 Gyr is about 13.7 Gyr, and abundant C A relatively narrow time window exists for geogenesis, betw Existence of and ability for dormancyMany up potential to habitats mi for life exist outside of Earth with Exchange of material within inner Solar SystemDisputed well-docume evidence for , including , , Triton, ) and, by inference, outsi • • • • • Source: Wikipedia † e Lattimer, AST 248, Lecture 13 – p.20/20 ing unsupported . impler hypothesis: g a singular event, or hout universe by t DNA of 1.1 million years c annihilation or to umanoid and living on xplanation for the ved his support in favor of eeding life is to randomly send class M) planets (a result of genetic codes spread through th i.e.,

Counterevidence Against Panspermia terraform planets for later colonization. Crick later remo small grains containing DNA. Motivation: avoid catastropi the RNA world hypothesis for life’s origin. Space is a damaging environment (cosmic rays,Studies extreme of cold) ice core bacteria indicate a half-life for dorman Occam’s Razor says when developing an hypothesis, avoid mak Crick and Orgel proposed life is deliberately spread throug Abundant fiction rationalizations, including the e in a radiation-free environment. assumptions. On this basis, geogenesislife is originates supported as as a the matterEarth s of does probability not as meet opposed proper to conditions. bein intelligent civilizations. Cost-effective strategy for s improbable tendency for fictional extraterrestrialssimilar to ( be h Universe by the “Ancient ”. • • • • •